1. Field of the Invention
The present invention relates to a multi-cylinder engine, and particularly to a multi-cylinder engine in which an intake valve and a first spark plug arranged side by side along an axis of a camshaft and an exhaust valve and a second spark plug arranged side by side along the axis of the camshaft are disposed in a cylinder head for every combustion chamber.
2. Description of the Related Art
Such engine is already well-known, for example, from Japanese Patent Publication No.60-10163.
In the conventional multi-cylinder engine, however, the position of a central portion of an upstream end of an intake port and the position of a central portion of a downstream end of an exhaust port are offset from each other in a direction along the axis of the camshaft, so that the size of the cylinder head in the direction along the axis of the camshaft inevitably increases.
Accordingly, it is an object of the present invention to provide a multi-cylinder, wherein the size of the cylinder head can be reduced in the direction along the axis of the camshaft.
To achieve the above object, according to a first feature of the present invention, there is provided a multi-cylinder engine in which an intake valve and a first spark plug arranged side by side along an axis of a camshaft and an exhaust valve and a second spark plug arranged side by side along the axis of the camshaft are disposed in a cylinder head for every combustion chamber, wherein the position of a central portion of an upstream end of an intake port provided in the cylinder head with the intake valve interposed between the intake port and each of the combustion chambers and the position of a central portion of a downstream end of an exhaust port provided in the cylinder head with the exhaust valve interposed between the exhaust port and each of the combustion chambers are set to correspond with each other in the direction along the axis of the camshaft.
With such arrangement of the first feature, the central portion of the upstream end of the intake port and the central portion of the downstream end of the exhaust port are disposed in the same position in the direction along the axis of the camshaft and hence, the distance between the cylinders can be determined at a small value, thereby reducing the size of the cylinder head in the direction along the axis of the camshaft.
According to a second feature of the present invention, in addition to the arrangement of the first feature, an EGR passage for guiding a portion of an exhaust gas flowing through the exhaust port is provided in the cylinder head at one end along the axis of the camshaft; the second spark plug is disposed at a location where the exhaust port is interposed between the second spark plug and the EGR passage; and the intake port is provided in the cylinder head at a location where the first spark plug is interposed between the intake port and the EGR passage. With such arrangement, it is possible to inhibit an adverse effect due to heat from the EGR passage to air flowing through the intake port, while simplifying the structure of communication between the exhaust port and the EGR passage.
According to a third feature of the present invention, in addition to the arrangement of the first or second feature, an EGR valve for controlling the flow of an EGR gas is mounted to a sidewall of the cylinder head into which the upstream end of the intake port opens, and the intake port is provided in the cylinder head at a location where the first spark plug is interposed between the intake port and the EGR valve. With such arrangement, it is possible to inhibit an adverse effect due to heat from the EGR passage to air flowing through the intake port.
According to a fourth feature of the present invention, in addition to the arrangement of the first or second feature, the first spark plug arranged side by side with the upstream end of the exhaust port in the direction perpendicular to the axis of the camshaft is disposed side by side with the intake valve along the axis of the camshaft; the second spark plug arranged side by side with the downstream end of the intake port in the direction perpendicular to the axis of the camshaft is disposed side by side with the exhaust valve along the axis of the camshaft; the upstream end of the intake port is disposed offset in one direction from the downstream end of the intake port in the direction along the axis of the camshaft; and the downstream end of the exhaust port is disposed offset in the other direction from the upstream end of the exhaust port in the direction along the axis of the camshaft. With such arrangement, an intake air flow from the intake port produces a swirl flow within the combustion chamber, while the area of opening of the downstream end of the intake port into the combustion chamber and the area of opening of the upstream end of the exhaust port into the combustion chamber are secured at large values to the utmost, thereby enhancing the combustion efficiency.
According to a fifth feature of the present invention, in addition to the arrangement of the first feature, a protrusion is integrally provided on the cylinder head to project outwards from a cylinder block; the first spark plug is mounted in the cylinder head with its axis disposed in a plane perpendicular to the axis of the camshaft; spark plug chambers corresponding to the combustion chambers and each faced by a portion of each of the first spark plugs are defined in the cylinder head in such a manner that a portion of each of the spark plug chambers is disposed in the protrusion; and a plurality of drainage bores are provided in the protrusion in such a manner that one end thereof opens into a lower portion of each of the spark plug chambers, and the other end thereof opens into an outer surface of a lower portion of the protrusion.
With such arrangement of the fifth feature, since the spark plug chambers each having a portion disposed in the protrusion are provided in the cylinder head and hence, the volume of each spark plug chamber can be set at a relatively large value to provide a reduction in weight of the cylinder head. Moreover, since the drainage bores are provided in the cylinder head, with one end opening into the lower portion of each of the spark plug chambers and with the other end opening into the outer surface of the lower portion of the protrusion, i.e., into an outer surface of a lower portion of the cylinder head outside the cylinder block, water entering the spark plug chambers can be discharged reliably by the short and simplified drainage bores.
According to a sixth feature of the present invention, in addition to the arrangement of the fifth feature, a fastening boss for fastening an intake device to the protrusion is provided on the cylinder head to protrude into a lower portion of one of the spark plug chambers, and one end of the drainage bore corresponding to the one spark plug chamber opens into the lower portion of the one spark plug chamber between the first spark plug and the fastening boss in the direction along the axis of the camshaft. With such arrangement, if the fastening boss for fastening the intake device is offset from the spark plug chamber, then an increase in size of the cylinder head is inevitable, but the increase in size of the cylinder head can be avoided by ensuring that the fastening boss protrudes into the lower portion of the one spark plug chamber. Moreover, water entering the spark plug chambers from around the spark plugs can be guided toward the drainage bores in such a manner that it is not obstructed by the fastening boss.
According to a seventh feature of the present invention, in addition to the arrangement of the fifth feature, a fastening boss for fastening an intake device to the protrusion is provided on the cylinder head to protrude into a lower portion of one of the spark plug chambers; one end of the drainage bore corresponding to the one spark plug chamber opens into the lower portion of the one spark plug chamber at a location where the fastening boss is interposed between the drainage bore and the first sparkplug in the direction along the axis of the camshaft; and a guide wall for guiding water entering around the first spark plug toward the drainage bore is provided between an inner wall of the spark plug chamber and the fastening boss. With such arrangement, if the fastening boss for fastening the intake device is offset from the spark plug chamber, then an increase in size of the cylinder head is inevitable, but the increase in size of the cylinder head can be avoided by ensuring that the fastening boss protrudes into the lower portion of the one spark plug chamber. Moreover, water entering the spark plug chambers from around the spark plugs can be guided by the guide wall toward the drainage bores in such a manner that it is not obstructed by the fastening boss. In addition, the rigidity of the fastening boss can be increased by the guide wall.
According to an eighth feature of the present invention, in addition to the arrangement of the fifth feature, at least a lower portion of an inner surface of one of the spark plug chambers is formed as a flat face parallel to the camshaft on the protruding side of the protrusion, and a threaded bore for threaded engagement with a fastening bolt for fastening an intake device is provided to extend between protruding end face of the protrusion and the flat face. With such arrangement, if the fastening boss for fastening the intake device is offset from the spark plug chamber, then an increase in size of the cylinder head is inevitable, but the increase in size of the cylinder head can be avoided by disposing the fastening boss in correspondence to the one spark plug chamber, and water entering the spark plug chambers from around the first spark plugs can be guided toward the drainage bores by the flat face in such a manner that it is not obstructed by the fastening boss.
According to a ninth feature of the present invention, in addition to the arrangement of the first feature, bosses for mounting fuel injection valves are provided on the cylinder head; a second EGR passage for guiding an EGR gas is provided in the cylinder head to extend in parallel to the axis of the camshaft in proximity to the fuel injection valves in such a manner that a portion of each of the bosses protrudes into the second EGR passage. With such arrangement, a portion of each of the bosses for mounting the fuel injection valves protrudes into the second EGR passage and hence, the second EGR passage can be provided in the cylinder head in proximity to the fuel injection valves, while ensuring a wall thickness of the bosses on the side of the second EGR passage. In addition, the rigidity of the bosses can be ensured, and the second EGR passage can be provided in proximity to the fuel injection valves so as to prevent an adverse effect due to a high-temperature EGR gas flowing through the second EGR passage from being exerted to the fuel injection valves, thereby providing the compactness of the cylinder head.
According to a tenth feature of the present invention, in addition to the arrangement of the ninth feature, the second EGR passage is disposed between the intake ports and the fuel injection valves. With such arrangement, the second EGR passage can be disposed effectively in a space between the intake ports and the fuel injection valves, thereby further contributing to the compactness of the cylinder head.
According to an eleventh feature of the present invention, in addition to the arrangement of the ninth or tenth feature, at least one of a plurality of spark plug chambers formed in the cylinder head in correspondence to the combustion chambers with a portion of each of the spark plugs being exposed thereto is disposed between the second EGR passage and the combustion chambers. With such arrangement, a reduction in weight of the cylinder head can be achieved by defining the spark plug chambers which are cavities, and the spark plugs can be disposed utilizing such cavities.